Active matrix liquid crystal display devices including a plurality of pixels arranged in matrix have been widespread. In general, the pixel includes a transistor having a gate electrically connected to a scan line and a source and a drain one of which is electrically connected to a signal line, a capacitor having terminals one of which is electrically connected to the other of the source and drain of the transistor and the other of which is electrically connected to a wiring supplying a common potential (hereinafter, also referred to as a capacitor wiring), and a liquid crystal element having terminals one of which (a pixel electrode) is electrically connected to the other of the source and the drain of the transistor and the one of the terminals of the capacitor and the other of which (a counter electrode) is electrically connected to a wiring supplying a counter potential.
An example of a structure of the above-described pixel is illustrated in FIGS. 13A to 13C. FIG. 13A is a top view of the pixel. Note that FIGS. 13A to 13C are diagrams in which parts (a liquid crystal layer, the counter electrode, and the like) of the liquid crystal element are omitted (a so-called active matrix substrate is illustrated). A pixel 1000 illustrated in FIG. 13A is provided in a region surrounded by a scan line 1001 and a scan line 1002 which are arranged in parallel or substantially parallel to each other and a signal line 1003 and a signal line 1004 which are arranged perpendicularly or substantially perpendicularly to the scan lines 1001 and 1002. Further, the pixel 1000 includes a transistor 1005, a capacitor 1006, and a pixel electrode layer 1007. Note that a conductive layer (a capacitor wiring 1008) which is to be one of electrode layers of the capacitor 1006 is arranged in parallel or substantially parallel to the scan lines 1001 and 1002 and is provided so as to be across the plurality of pixels.
FIG. 13B is a cross-sectional view taken along line A-B in FIG. 13A. The transistor 1005 includes a gate layer 1011 provided over a substrate 1010, a gate insulating layer 1012 provided over the gate layer 1011, a semiconductor layer 1013 provided over the gate insulating layer 1012, one of a source layer and a drain layer 1014a provided over one end of the semiconductor layer 1013, and the other of the source and drain layers 1014b provided over the other end of the semiconductor layer 1013. The capacitor 1006 includes part of the capacitor wiring 1008, an insulating layer (the gate insulating layer 1012) provided over the capacitor wiring 1008, and the other of the source and drain layers 1014b provided over the insulating layer. In addition, the other of the source and drain layers 1014b is electrically connected to the pixel electrode layer 1007 in a contact hole 1016 formed in an insulating layer 1015 provided over the transistor 1005 and the capacitor 1006.
FIG. 13C is a cross-sectional view taken along line C-D in FIG. 13A. The signal line 1003 intersects with the scan line 1001, the capacitor wiring 1008, and the scan line 1002 in a region 1017a, a region 1017b, and a region 1017c respectively with the gate insulating layer 1012 interposed therebetween. Therefore, an upper surface of the signal line 1003 has a convex shape in the regions 1017a, 1017b, and 1017c. Note that it is apparent that the signal line 1004 also has the same upper surface shape as the signal line 1003.
Note that in a liquid crystal display device including the pixel 1000 illustrated in FIGS. 13A to 13C, the scan lines 1001 and 1002 and the capacitor wiring 1008 are formed using the same conductive film, and the gate insulating layer 1012 in the transistor 1005 is also used as a dielectric in the capacitor 1006. That is, it can be said that the liquid crystal display device is a liquid crystal display device whose manufacturing process steps are reduced.
In the pixel 1000 illustrated in FIGS. 13A to 13C, the transistor 1005 has a function of controlling input of a data signal which determines a voltage applied to the liquid crystal element (a potential applied to the pixel electrode layer 1007), and the capacitor 1006 has a function of holding the voltage applied to the liquid crystal element (the potential applied to the pixel electrode layer 1007).
For example, in the case where the dielectric of the capacitor 1006 is formed with a silicon oxide film with a thickness of 0.1 μm, the area of the capacitor 1006 having a capacitance of 0.4 pF is approximately 1160 μm2. Here, when the size of the pixel is 42 μm×126 μm (a 4-inch VGA pixel), the proportion of the area of the capacitor 1006 to the pixel is approximately 22%, which causes a reduction in the aperture ratio. Note that the capacitor 1006 can be eliminated in the above pixel structure. A certain amount of charge can be held without intentionally providing the capacitor 1006 because the liquid crystal element itself has storage capacitance. However, the relative permittivity of liquid crystal is about 3 at the lowest, and the cell gap is 3 μm to 4 μm. Consequently, electrostatic capacitance is approximately 1/50 of that of the device using the capacitor 1006 having a 0.1-μm-thick silicon oxide film as a dielectric, and therefore, the area of the liquid crystal element is required to be approximately 58000 μm2. Since this size is comparable to that of the pixel with a size of 140 μm×420 μm, the resolution is reduced to approximately 60 ppi and charge can be held only when the liquid crystal display devices have a resolution of 60 ppi or lower. In other words, when pixels are formed with a resolution of 60 ppi or more, the capacitor 1006 is required.
In the liquid crystal display device, by controlling a potential of the scan line 1001, the transistor 1005 is turned on and a potential of the signal line 1003 is controlled as a data signal for the pixel 1000. Thus, a desired voltage can be applied to the liquid crystal element included in the pixel 1000. The voltage is held by the capacitor 1006 for a certain period, so that desired display can be performed in each pixel for a certain period. The liquid crystal display device successively performs such operation for each pixel, whereby images (still images) are formed in a pixel portion. Further, the liquid crystal display device displays a moving image by changing the images successively (e.g., 60 times per second (at a frame frequency of 60 Hz)).
As described above, the moving image is formed of many still images. That is, strictly speaking, the moving image is not a continuous image. Accordingly, when fast moving images are displayed, residual images are readily generated in display. In particular, in a liquid crystal display device, each pixel maintains display from when a data signal is input to the pixel to when the next data signal is input to the pixel; therefore, residual images tend to be apparent. In Patent Document 1, a technique to reduce residual images (referred to as “double-frame rate driving” in general) is disclosed. Specifically, in Patent Document 1, the following technique is disclosed: an image for interpolation is formed between two images displayed sequentially, and the image is inserted between two images displayed sequentially, so that residual images are reduced.